Generally, a relative revolutionary sliding movement occurs between a pin and a link plate in case of a silent chain and between a pin and a bush in case of a roller chain, thus causing abrasion of the pin. Then, various surface treatments have been implemented on the surface of the pin in order to reduce such abrasion.
Conventionally, as a pin whose surface is treated, there have been known a pin in which a chrome carbide layer is formed on the surface thereof (referred to as a chromizing pin hereinafter) and a pin in which a vanadium carbide layer is formed on the surface thereof (referred to as a VC pin hereinafter). Because there are cases when the chromizing pin causes peeling at the surface of the chrome carbide layer and when the VC pin causes peeling at the boundary surface between the vanadium carbide layer and a base material (raw material of the pin) under a use environment in which high surface pressure is repeatedly applied, it is considered that resistance of chrome carbide to surface pressure is low even though its adhesion (binding quality) with the base material is good and that resistance of vanadium carbide to surface pressure is high even though its adhesion with the base material is low.
Based on the result of the above consideration, Japanese Patent Laid-open No. 2002-195356 has proposed a method of forming a chrome carbide layer on the surface of a pin base material made from steel by carrying out a chrome cementation process and of forming, thereon, a mixed layer which is thicker than the above-mentioned chrome carbide layer and which contains vanadium carbide as its main component and a small amount of chrome carbide by carrying out a vanadium cementation process at temperature higher than that of the chrome cementation process for the purpose of improving the abrasion resistance under high surface pressure.
However, the pin proposed as described above has had a problem that because the chrome and vanadium cementation process has to be carried out after carrying out the chrome cementation process at the temperature higher than that of the chrome cementation process, it has been cumbersome and costly to carry out such surface treatments.
Especially, in case of a timing chain or the like to which high surface pressure is applied under a high temperature environment within an engine, the surface pressure resistance of the outermost surface of the pin is not enough as compared to one composed of vanadium carbide (Vx Cy, V8C7 for example) because it is formed of the mixed layer of vanadium carbide and chrome carbide [(V, Cr)xCy, (V, Cr) 8C7 for example]. Thus it may cause peeling on the surface and abrasion thereof may advance with the advance of peeling. Still more, because a clear intermediate layer composed of the chrome carbide layer exists between the chrome and vanadium mixed layer and the steel, i.e., the base material, there is a possibility of causing peeling at the boundary surfaces between the intermediate layer and the mixed layer, i.e., the upper layer thereof, and between the intermediate layer and the steel base material, i.e., the lower layer thereof, causing quick abrasion as a result.